1. Field of the Invention
The present invention generally relates to relatively electrical small antennas. More particularly, the present invention relates to antenna structures for mobile communication devices having constraints on internal space and battery consumption.
2. Discussion of Background
There has been increasing need for innovative antennas on mobile terminals in wireless communications including the global system for mobile communications (GSM850) or extended GSM (EGSM), the digital communication system (DCS), the personal communication system (PCS), and wide-band code-division multiple access (WCDMA). Constraints on such antenna design include requirements of multiband or broadband resonance, limitation of space in handheld devices, reduction of radio absorption in the user's head or body for antenna efficiency and safety measures, and cost reduction. Traditional antennas such as monopoles, dipoles and even patches are unable to meet these requirements and hence alternative approaches are needed.
Planar antennas have features of low cost, low profile and light weight. However, a planar antenna performance is related to the shape and dimensions of the antenna wires and slits or slots on ground planes and have quite narrow bandwidth.
FIG. 1 (PRIOR ART) is a drawing of a known basic model of a planar inverted-F antenna (PIFA) antenna 100 comprising a planar electrically conductive radiating element 101, electrically conductive ground plane 102 parallel to the radiating element 101, and, connecting these two, a ground contact 103 which is substantially perpendicular to the radiating element 101 and ground plane 102. The structure further includes a feed electrode 104, which also is substantially perpendicular to the radiating element 101 and ground plane 102 and which can be coupled to an antenna port (not shown) of a radio apparatus. In the structure of FIG. 1, the radiating element 101, ground contact 103 and the feed electrode 104 are usually manufactured by cutting or stamping a thin metal sheet into a suitable shape, which is conformable to the housing to some degree and which has two protrusions bent to a right angle. The ground plane 102 may be composed of a metallized area on the surface of a printed circuit board (PCB) so that the ground contact 103 and the feed electrode 104 are easily connected to electrodes on the printed circuit board. The electrical characteristics of the antenna 100 are affected in general by the dimensions of its elements and, in particular, by the size of the radiating element 101 and its distance from the ground plane 102.
FIG. 2 (PRIOR ART) is a drawing of a PIFA structure 200 according to European Patent document No. 484,454, wherein a radiating element 201, ground plane 202 and a ground contact 203 connecting these two are realized as metal platings on surfaces of a solid dielectric body 204. The antenna is fed through a coupling element 205, which does not touch the radiating element 201. An electromagnetic coupling exists between the coupling element 205 and radiating element 201, and the coupling element 205 extends over the edge of the dielectric body 204 to a point that can be coupled to the antenna port of a radio apparatus. The structure is mechanically sturdy, but the dielectric body block makes it relatively heavy. Further, the dielectric body makes the impedance bandwidth of the antenna narrower and degrades the radiation efficiency as compared to an air-insulated PIFA structure.
FIG. 3 (PRIOR ART) is a drawing of known PIFA structure 300 having a known design of a PIFA radiating element 301. The rectangular shape is broken by a gap 302, which forms a strip in that portion of the radiating element which is farthest away from the feed point 303 and ground contact 304. The purpose of the gap typically is to increase the electrical length of the antenna and, thereby, to affect the antenna's resonating frequency.
All the PIFA structures described above are designed such that they have a certain resonating frequency, as well as an operating frequency band centered around the resonating frequency. However, these PIFA structures are not designed to fit in a small confined space while communicating efficiently in a wide frequency band.
What is needed is an antenna that can fit in a relatively small confined space while communicating efficiently in a broadband network.